1. Field of the Invention
The present invention relates to a red-emitting organic electroluminescent compound. In particular, the invention involves a red-emitting organic electroluminescent compound that is capable of increasing the purity of red light.
2. Description of the Prior Art
In recent years, organic electroluminescent devices (OELDs) have been developed to become highly efficient and capable of producing a wide range of colors. As a result, the scope of OELD application has become even more versatile, such as flat panel electronic displays.
Organic light emitting devices (OLEDs) employ electroluminescence induced by applying a voltage to inject carriers to organic material that is luminescent. In FIG. 1, a basic OLED device is composed of an organic and luminescent material 4 formed between an upper metal cathode 2 and a lower transparent anode 6. The entire device is formed on a transparent substrate 8 (such as glass), and the transparent anode 6 is usually conductive ITO (Indium Oxide doped with Tin). FIG. 2 shows the energy gap of the device shown in FIG. 1. When a forward bias is applied between the anode 6 and the cathode 2, holes (+) and electrons (−) are injected to the organic semiconductor from anode 6 and cathode 2. Two carriers meet while transported in the organic thin film. Photons are produced by radiative recombination, thus the device emits light 10 via transparent anode and transparent substrate. In 1987, Kodak Co. proposed a breakthrough double layer device, shown in FIG. 3. 12 represents a metal cathode (MgAg), 14 represents an electron-transport layer (Alq3), 16 represents a hole-transport layer (diamine), 18 represents a transparent anode (ITO), and 20 represents a transparent substrate (glass). Structural formulas for Alq3 of the electron-transport layer 14 and diamine of the electron hole-transport layer 16 are respectively shown in FIGS. 4a and 4b. A device having this structure allows for separation of various functions on different layers. For instance, hole-transport layer 16 is in charge of injecting holes and transporting, and electron-transport layer 14 is in charge of injecting electrons and transporting. It is observed from the energy gap of the double-layer device shown in FIG. 5 that spacial distribution of carriers is limited by energy barriers located at heterojunctions, thereby increasing the frequency for combining carrier radiation. The double-layer device of Kodak has been extended further to multi-layer structure, where injecting electrons, transporting electrons, injecting holes, transporting holes and light emission caused by combining carrier radiation are spreaded on different material layers.
In earlier times, organic electroluminescent material used in OLED was mainly conjugated from organic host materials and conjugated organic activating agent having condensed benzene rings. However, researchers have found that doping a small amount of highly effective emissive dopant in the electroluminescent host material improves the efficiency for radiative combination of carriers. That is, doping various colours, such as red, green, and blue organic electroluminescent material in organic host material to obtain required electroluminescence.
However, for current organic electroluminescent technology, efficiency and purity for red light are the lowest, since they deviate to yellow or orange. For example, U.S. Pat. No. 5,935,720 discloses a red-emitting material DCJTB with λ em=615 nm (as shown in FIG. 1 of the patent), consequently its light appears orange. Furthermore, efficiency is lowered due to concentration quenching effect. Although there has been a proposal for phosphorous red-emitting material, it is not ideal in terms of efficiency due to longer decay time and lower thermal stability. As a result, it is critical to develop red-emitting material for organic electroluminescent elements.